The present invention relates to a process for producing halogenated phthalic anhydride. More particularly, it relates to a process for halogenating, especially chlorinating, phthalic anhydride in a vapor phase at elevated temperature.
Halogenated phthalic anhydride is utilized as an intermediate for the synthesis of medicines, agricultural chemicals and synthetic resin materials. Usually halogenated phthalic anhydride is produced by a process in which phthalic anhydride is dissolved in an alkaline aqueous solution to form an alkali metal salt of phthalic acid and halogen gas is introduced into the aqueous solution for halogenation.
According to this method, however, since halogenation is carried out in aqueous solution, and halogenated phthalic acid is produced as reaction product, it is necessary to carry out a separation of the halogenated phthalic acid from the aqueous solution and a dehydration reaction of the halogenated phthalic acid into halogenated phthalic anhydride. Therefore, this process is complicated and is time-consuming, and also the treatment of the waste water discharged from the separation step in large quantities, is very troublesome. Further, the result of the halogenation reaction itself is not well satisfactory. Methods for chlorinating phthalic anhydride in a vapor phase without using a catalyst have been known (U.S. Pat. No. 4,297,283), but these methods involved the serious problems such as corrosion of the apparatus since the reaction is carried out at a high temperature.
In view of the above circumstances, the present inventors gave their attention on the fact that vapor-phase halogenation of phthalic anhydride is advantageous over liquid-phase reaction as the reaction process is greatly simplified and also troublesome waste water is not generated. However, the conventional non-catalytic vapor-phase reactions are carried out at a very high temperature which is above 400.degree. C. Also, the desired halogenated product can hardly be obtained when the reaction is carried out at a low temperature, especially below 350.degree. C. The present invention, therefore, is intended to provide a process for halogenating phthalic anhydride in a vapor phase at a relatively low temperature.
As a result of the present inventors' strenuous researches for solving the above technical problems, it has been found that by reacting phthalic anhydride and a molecular halogen in a vapor phase in the presence of a catalyst containing zeolite as an active component, it is possible to remarkably simplify the reaction process and to produce halogenated phthalic anhydride efficiently at a relatively low temperature without dehydration and discharge of a large quantity of waste water. The present invention has been attained on the basis of this finding.